Conventionally, ironworks produce steel plates of various thicknesses and sizes by a hot rolling process (also referred to as a hot strip mill process). The hot rolling process is a process of sandwiching a thick steel plate called a slab from above and below by rollers and stretching the slab to produce a thin plate called a coil.
Surface quality of the coil thus produced depends on a surface state of the rollers. When a plurality of slabs are rolled one after another by the rollers, the surface state of the rollers deteriorates gradually. Therefore, a slab required to have high quality is desirably rolled while the rollers are still new.
Moreover, rolling a slab of a certain width may leave a groove as wide as the slab, on a roller surface. This being so, a groove left on a roller surface by a narrow slab rolled earlier may cause a flaw on a wide slab rolled later. Furthermore, in the case of successively rolling two slabs that differ in thickness, the difference in thickness is limited to a predetermined range according to specifications of rolling equipment and the like. Besides, to prevent a decrease in roller durability, thin slabs in particular cannot be successively rolled in large numbers. Thus, various constraints on an order in which slabs are rolled need to be satisfied to maintain slab quality and also improve productivity.
FIG. 1 is a view showing flows of slabs to a hot rolling mill for performing a typical hot rolling process. In detail, as shown in FIG. 1, slabs cast by a direct slab caster 104 are directly fed to a hot rolling mill 102. Here, since the slabs are fed to the hot rolling mill 102 in sequence, it is impossible to change an order in which the slabs are arranged. Note that slabs 106 supplied from the direct slab caster 104 are called direct slabs or DHCR (direct hot charge rolling) slabs.
In general, a hot rolling process in the hot rolling mill 102 is faster than a steelmaking process in the direct slab caster 104. Accordingly, inventory slabs 110 are prepared in a slab yard 108 beforehand, and the direct slabs 106 and the inventory slabs 110 are fed to the hot rolling mill 102 while being combined appropriately, thereby achieving efficient use of the hot rolling mill 102. The slabs rolled in the hot rolling mill 102 are then stored as coils 120.
Note that the inventory slabs 110 are also called HCR/CCR slabs. HCR stands for hot charge rolling and is also called a hot strip, and CCR stands for cold charge rolling and is also called a cold strip.
The reason for calling the inventory slabs 110 by such a mixed name as HCR/CCR slabs is that there are two flows to the inventory slabs 110, namely, a flow of slabs from an inventory slab caster 112 and a flow of slabs from the direct slab caster 104. Of the inventory slabs 110, a HCR slab is a slab that is sent to the hot rolling mill 102 within a predetermined time (24 hours as an example) after tapping, before cooling down sufficiently. A CCR slab, on the other hand, is a slab that has been left for at least five days as an example and has cooled down sufficiently.
In either case, the inventory slabs 110 are too low in temperature to be directly fed to the hot rolling mill 102. Therefore, after scheduling an order of HCR slabs and CCR slabs, these slabs need to be heated in a reheating furnace 114 before being fed to the hot rolling mill 102.
Japanese Unexamined Patent Publication No. 2007-222911 by the applicant of this application discloses a technique of determining a processing operation arrangement order that maximizes slab processing efficiency, especially by solving an integer programming problem. This technique can be applied to scheduling of an order of HCR slabs and CCR slabs in the inventory slabs 110.
Japanese Unexamined Patent Publication No. 2000-167610 relates to a rolling order determination method and a rolling order determination apparatus in hot rolling, and discloses a new rolling order determination method that can optimally schedule heating and rolling while attaching importance to grouping of rolled materials having the same heating condition.
Japanese Unexamined Patent Publication No. 2004-209495 discloses a scheduling method for maximizing a processing capacity in hot rolling in consideration of capacity synchronization between a heating furnace and a rolling mill, rolling constraints, heating constraints, and electric power costs.
Japanese Unexamined Patent Publication No. 2003-305508 relates to a rolling procedure determination method, and discloses the following method. Information including an occupancy time of a rolling mill when rolling is performed is obtained based on a pass schedule of each material to be rolled. With a rolling completion time by which rolling of all materials rolled after passing through a heating furnace is completed being set as a target based on the obtained information, constraints such as an inability to simultaneously perform rolling in the rolling mill are formulated to an optimization problem, the formulated optimization problem is converted to a mixed integer programming problem, and an order of extraction of materials from the heating furnace and an order of rolling in the rolling mill are determined so as to minimize the rolling completion time.
Japanese Unexamined Patent Publication No. 2005-342787 discloses a combination of a schedule target steel information processing apparatus, a simulation apparatus, and an optimal schedule calculation apparatus, where the schedule target steel information processing apparatus stores information about schedule target steel and a physical distribution state of schedule target steel from a steel mill to a hot rolling mill and automatically updates the information via a network, the simulation apparatus receives a current status of schedule target steel from the schedule target steel information processing apparatus and predicts a future physical distribution state, and the optimal schedule calculation apparatus calculates a schedule instruction according to an operation progress status in the hot rolling mill and a future hot rolling arrival state of schedule target steel predicted by the simulation apparatus.